1. Field of the Invention
The present invention relates to a steering stability control system for a vehicle, comprising a turning state judging means for judging the turning state of the vehicle to deliver an oversteering signal or an understeering signal, and a steering stability control means for controlling the output torque from an engine on the basis of the output from the turning state judging means.
2. Description of the Related Art
There is a conventionally known steering stability control system for a vehicle, which is designed such that a reference yaw rate to be intrinsically generated by the vehicle in accordance With the operational condition of the vehicle is compared with an actual yaw rate actually generated; the turning state of the vehicle is judged, i.e., it is judged whether the vehicle is in an oversteering or in an understeering, and the output torque from the engine is increased or reduced in accordance with the judged turning state, thereby compensating for the oversteering and the understeering to properly maintain the steering stability of the vehicle.
This steering stability control system will be described below with reference to FIGS. 4 and 5. For example, it is considered that the vehicle is turned in a clockwise direction through a path (1) to (6), as shown in FIG. 4. FIG. 5 illustrates the variations in steering angle .delta. of a steering wheel, reference yaw rate Y.sub.REF and actual yaw rate Y. Suppose that when a driver of the vehicle steers the steering wheel in a clockwise direction (which is referred to as a positive direction herein), so that the vehicle is turned in the clockwise direction through a path (1) to (3), an oversteering tendency is produced, causing the vehicle to run inwardly in the turning direction. The driver counter-steers the steering wheel in a counterclockwise direction (which is referred to as a negative direction herein) from a position indicated by (3) for the purpose of maintaining the turning radius constant. This causes the steering angle .delta. to become 0 (zero) at a position indicated by (4) and further to become the negative maximum value at a position indicated by (5). At a position (6) in which the turning is completed, the steering angle .delta. becomes 0 (zero) again. At that time, the reference yaw rate Y.sub.REF determined on the steering angle .delta. follows the variation in steering angle .delta. with a slight delay due to an influence of a play of a steering system or the like. The actual yaw rate Y which is an actual yaw rate of the vehicle does not immediately follow the counter-steering operation due to an inertia of the vehicle and is maintained positive with the exception of the last stage of the turning.
Table 1 illustrates the criterion for prior art judgement of an oversteering and an understeering of the vehicle. The positive and negative of the reference yaw rate Y.sub.REF (the axis of ordinates) are compared with the positive and negative of a deviation Y--Y.sub.REF (the axis of abscissas) resulting from subtraction of the reference yaw rate Y.sub.REF from the actual yaw rate Y, and the oversteering and the understeering can be judged by a combination of these positives and negatives.
TABLE I ______________________________________ Actual yaw rate-ref. yaw rate Reference yaw rate non-negative negative ______________________________________ non-negative: oversteering understeering negative: understeering oversteering ______________________________________
If Table 1 is applied to a graph shown in FIG. 5, in a region (A) of regions (A) and (B) in which the reference yaw rate Y.sub.REF is non-negative, the deivation Y--Y.sub.REF resulting from subtraction of the reference yaw rate Y.sub.REF from the actual yaw gate Y is negative. Therefore, the region (A) corresponds to a right and upper column, and in the region (A), it is judged that the vehicle is in the understeering. On the other hand, in the region (B), the deviation Y--Y.sub.REF resulting from substraction of the reference yaw rate Y.sub.REF from the actual yaw rate Y is non-negative. Therefore, the region (B) corresponds to a left and upper column, and in the region (B), it is judged that the vehicle is in the oversteering. In a region (C), the reference yaw rate Y.sub.REF is negative and the deviation Y--Y.sub.REF resulting from subtraction of the reference yaw rate Y.sub.REF from the actual yaw rate Y is non-negative. Therefore, the region (C) corresponds to a left and lower column, and in the region (C), it is judged that the vehicle is in the understeering.
If it is judged that the vehicle is in the oversteering or the understeering, the output torque from the engine in the front wheel drive vehicle can be increased or reduced, thereby avoiding the turning of the vehicle in an undesirable direction.
With the above-described prior art technique for judging the oversteering and understeering, however, it is misjudged in the region (C) in FIG. 5 that the vehicle is in the understeering, notwithstanding that the counter-steering is conducted, because the vehicle shows an oversteering tendency. Thus, the output torque from the engine should be intrinsically increased, but a control is performed so that the output torque from the engine is reduced inversely.